Subject: Re: [RFC bpf-next 07/16] kallsyms: Use rb tree for kallsyms name search

From: Jiri Olsa <[email-masked]>
Date: Wed, 28 Oct 2020 19:25:34 +0100

On Thu, Oct 22, 2020 at 10:21:29AM +0200, Jiri Olsa wrote:
> The kallsyms_expand_symbol function showed in several bpf related
> profiles, because it's doing linear search.
> 
> Before:
> 
>  Performance counter stats for './src/bpftrace -ve kfunc:__x64_sys_s* \
>    { printf("test\n"); } i:ms:10 { printf("exit\n"); exit();}' (5 runs):
> 
>      2,535,458,767      cycles:k                         ( +-  0.55% )
>        940,046,382      cycles:u                         ( +-  0.27% )
> 
>              33.60 +- 3.27 seconds time elapsed  ( +-  9.73% )
> 
> Loading all the vmlinux symbols in rbtree and and switch to rbtree
> search in kallsyms_lookup_name function to save few cycles and time.
> 
> After:
> 
>  Performance counter stats for './src/bpftrace -ve kfunc:__x64_sys_s* \
>    { printf("test\n"); } i:ms:10 { printf("exit\n"); exit();}' (5 runs):
> 
>      2,199,433,771      cycles:k                         ( +-  0.55% )
>        936,105,469      cycles:u                         ( +-  0.37% )
> 
>              26.48 +- 3.57 seconds time elapsed  ( +- 13.49% )
> 
> Each symbol takes 160 bytes, so for my .config I've got about 18 MBs
> used for 115285 symbols.
> 
> Signed-off-by: Jiri Olsa <[email-masked]>

FYI there's init_kprobes dependency on kallsyms_lookup_name in early
init call, so this won't work as it is :-\ will address this in v2

also I'll switch to sorted array and bsearch, because kallsyms is not
dynamically updated

jirka

> ---
>  kernel/kallsyms.c | 95 ++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 86 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 4fb15fa96734..107c8284170e 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -50,6 +50,36 @@ extern const u16 kallsyms_token_index[] __weak;
>  
>  extern const unsigned int kallsyms_markers[] __weak;
>  
> +static struct kmem_cache *symbol_cachep;
> +
> +struct symbol {
> +	char		name[KSYM_NAME_LEN];
> +	unsigned long	addr;
> +	struct rb_node	rb_node;
> +};
> +
> +static struct rb_root symbols_root = RB_ROOT;
> +
> +static struct symbol *find_symbol(const char *name)
> +{
> +	struct symbol *sym;
> +	struct rb_node *n;
> +	int err;
> +
> +	n = symbols_root.rb_node;
> +	while (n) {
> +		sym = rb_entry(n, struct symbol, rb_node);
> +		err = strcmp(name, sym->name);
> +		if (err < 0)
> +			n = n->rb_left;
> +		else if (err > 0)
> +			n = n->rb_right;
> +		else
> +			return sym;
> +	}
> +	return NULL;
> +}
> +
>  /*
>   * Expand a compressed symbol data into the resulting uncompressed string,
>   * if uncompressed string is too long (>= maxlen), it will be truncated,
> @@ -164,16 +194,12 @@ static unsigned long kallsyms_sym_address(int idx)
>  /* Lookup the address for this symbol. Returns 0 if not found. */
>  unsigned long kallsyms_lookup_name(const char *name)
>  {
> -	char namebuf[KSYM_NAME_LEN];
> -	unsigned long i;
> -	unsigned int off;
> +	struct symbol *sym;
>  
> -	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
> -		off = kallsyms_expand_symbol(off, namebuf, ARRAY_SIZE(namebuf));
> +	sym = find_symbol(name);
> +	if (sym)
> +		return sym->addr;
>  
> -		if (strcmp(namebuf, name) == 0)
> -			return kallsyms_sym_address(i);
> -	}
>  	return module_kallsyms_lookup_name(name);
>  }
>  
> @@ -743,9 +769,60 @@ static const struct proc_ops kallsyms_proc_ops = {
>  	.proc_release	= seq_release_private,
>  };
>  
> +static bool __init add_symbol(struct symbol *new)
> +{
> +	struct rb_node *parent = NULL;
> +	struct rb_node **p;
> +	struct symbol *sym;
> +	int err;
> +
> +	p = &symbols_root.rb_node;
> +
> +	while (*p != NULL) {
> +		parent = *p;
> +		sym = rb_entry(parent, struct symbol, rb_node);
> +		err = strcmp(new->name, sym->name);
> +		if (err < 0)
> +			p = &(*p)->rb_left;
> +		else if (err > 0)
> +			p = &(*p)->rb_right;
> +		else
> +			return false;
> +	}
> +
> +	rb_link_node(&new->rb_node, parent, p);
> +	rb_insert_color(&new->rb_node, &symbols_root);
> +	return true;
> +}
> +
> +static int __init kallsyms_name_search_init(void)
> +{
> +	bool sym_added = true;
> +	struct symbol *sym;
> +	unsigned int off;
> +	unsigned long i;
> +
> +	symbol_cachep = KMEM_CACHE(symbol, SLAB_PANIC|SLAB_ACCOUNT);
> +
> +	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
> +		if (sym_added) {
> +			sym = kmem_cache_alloc(symbol_cachep, GFP_KERNEL);
> +			if (!sym)
> +				return -ENOMEM;
> +		}
> +		off = kallsyms_expand_symbol(off, sym->name, ARRAY_SIZE(sym->name));
> +		sym->addr = kallsyms_sym_address(i);
> +		sym_added = add_symbol(sym);
> +	}
> +
> +	if (!sym_added)
> +		kmem_cache_free(symbol_cachep, sym);
> +	return 0;
> +}
> +
>  static int __init kallsyms_init(void)
>  {
>  	proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> -	return 0;
> +	return kallsyms_name_search_init();
>  }
>  device_initcall(kallsyms_init);
> -- 
> 2.26.2
>